Effects on the Immune System 391tant to THC-mediated proliferation when stimulated by their CD3 receptor. The
THC-induced enhancement appeared to be related in part to levels of interleukin
(IL)-2 since addition of this cytokine modified the THC-induced up-regulation of
cell proliferation. In contrast, cells from 18-month-old mice remained resistant to
modulation by THC. It was concluded that the difference in immune responsive-
ness to THC related to the age of mice correlated, at least in part, to IL-2 levels in
the 2-week-old and young adult mice. Ramarathinam et al. (1997) demonstrated
that lymphoid cells from young and old mice exhibited different immunological
potential in terms of ability to produce cytokines following stimulation with either
ConA or anti-CD3 antibody. Levels of the anti-inflammatory cytokines IL-4 and IL-
10 were up-regulated in spleen cell cultures from the older animals. Furthermore,
in vivo administration of THC resulted in an up-regulation of the proliferative
response of lymphoid cells from young adult mice. Such enhancement was not
evident for cells from older animals.
2.3
Effects on the Immune System Using In Vitro Models
2.3.1
Effects on Mixed Cell Populations
Early experiments involved the use of mixed cell populations, since these more
closely replicatedtheinvivoconditionofacomplexmixtureofdistinctive cell types
cross-talking through soluble mediators as well as interacting with each other
through cell-contact-dependent modalities. Furthermore, the use of mixed cell
populations lent itself to the application of depletion and reconstitution studies for
the definition of specific cell subpopulations affected by cannabinoids. Lefkowitz et
al. (1978) examined the effect of THC on the in vitro sensitization of mouse splenic
lymphocytes with sheep erythrocytes (SRBC) using a plaque-forming cell (PFC)
assay. Splenic lymphocytes from mice injected with THC showed a depressed im-
munological response when compared with those from control animals. A similar
alteration in the immunological response was obtained when THC was added
directly to the culture medium as demonstrated by a reduction in the number of
plaque-forming centers. Baczynsky et al. (1983b) reported that cannabinoids acted
differentially in their suppressant effect on immunocytes in vitro. They examined
the effects of THC, CBD, and CBN on the primary-like immune response in cultures
of mouse spleen cells. THC and CBD, but not CBN, depressed the primary-like im-
mune response of stimulated mouse splenocytes. It was noted that THC exerted
its maximal suppression of immune responses when administered antecedent to
antigenic stimulation. Pross et al. (1992) found that, when the mitogens ConA or
PHA were used to stimulate THC-treated splenocytes, a down-regulation of lym-
phocyte proliferation occurred. This down-regulation was accompanied by lower
T cell numbers in general and Ly2-positive cells specifically. In contrast, when
splenocytes were stimulated directly with anti-CD3 antibody, low concentrations
of THC enhanced lymphocyte proliferation that was accompanied by greater num-
bers of T cells in general and Ly2 cells specifically. In a subsequent study, Pross et